This invention relates to a method and apparatus for producing a high degree of vacuum (10.sup.-3 torr to 10.sup.-7 torr and even higher vacuums) utilizing only single-stage pumps. Currently, physical limitations of single-stage pumps preclude them from efficiently evacuating an enclosure to an internal pressure below approximately 10.sup.-2 torr. See, "Vacuum Techniques", Encyclopedia of Physics, 2d Ed., pp. 976.
Some methods available to achieve a high degree of vacuum utilize multi-stage pumps, cryopumps, turbomolecular pumps, rotary pumps with oil seals, and diffusion pumps (oil and mercury vapor). Cryopumps consist of one or more exposed surfaces refrigerated to a temperature usually below 100.degree. K., at which certain gases will be condensed and form a layer having an equilibrium vapor pressure below a specified limit. A diffusion pump utilizes one or more jets of vapor into which molecules from the chamber can diffuse and be carried forward into a region of higher pressure. Mercury vapor pumps require efficient refrigerated traps to keep mercury vapor out of the chamber being evacuated. Oil vapor jet pumps, while able to achieve high vacuum levels, cannot discharge directly to the atmosphere because of limitations on the vapor pressure which can be generated. See, "High Vacuum Pumps" by B. B. Dayton, Standard Handbook for Mechanical Engineers, 1978, pp 14-44-14-48 and "Vacuum Systems", Perry's Chemical Engineers' Handbook, 6th Ed., 1984, pp. 6-32-6-37. Single-stage pumps offer the advantages of fewer moving parts and seals, lower energy requirements, and lower attendant maintenance costs than the more-complicated systems currently available for achieving high vacuums.
Another approach has been to couple an airlock to an expansion tank having a volume larger than the airlock. Such a system is disclosed in U.S. Pat. No. 4,283,631. One of the difficulties in achieving a high degree of vacuum in the system disclosed in that patent is that the expansion tank is evacuated to the atmosphere. Conventionally, a single-stage pump evacuating a system to the atmosphere has a limited ability to evacuate enclosures below a vacuum of about 10.sup.-2 torr. Since the expansion tank disclosed in U.S. Pat. No. 4,283,631 is constantly evacuated to the atmosphere, the ultimate vacuum level of the airlock is dictated by the volume of the expansion tank. In order to get progressively higher vacuum levels in the airlock, the volume of the expansion tank must be increased proportionally. Accordingly, a single-stage pump would be impractical for evacuating the expansion tank and airlock to a sufficiently high degree.
It is an object of this invention to overcome these limitations inherent in vacuum pumping systems.
It is another object of the invention to provide a vacuum pumping system that achieves a high degree of vacuum using only a single-stage pump.